Is safe S/M role-playing normal? And where does one draw the line if mutual consent is established?
In S/M, or sadomasochistic sex play, the mutually agreed upon playacting is based on fantasy situations of dominance and submission. One partner will "force" her/his will on the other, consensually experimenting with activities that involve physical pain, discomfort, or intensity, until the other gives the signal to stop. S/M pushes the boundaries between pleasure and pain. To give you a short answer the question about S/M being "normal," it absolutely is. Consensual sexual behavior has many variations (including S/M), and it really comes down to what is right for you and your partner(s).
Mutual consent is what distinguishes S/M from abuse and assault, just as consent distinguishes sex from rape. S/M can encompass physical and/or psychological interactions, and may cause pain, but not physical or emotional damage. Accidents can happen during S/M, just as in any other physical activity, but this differs from abuse. For some, S/M play can increase sexual pleasure and open up hidden issues of power, which are always present in human intimacy. Trust plays a large part in S/M activity. Some refer to S/M as sensuality and mutuality. Partners need to talk with one another before they begin a "scene" to learn what each of them likes, would like to try, and would not like to do under any circumstances.
When engaging in S/M play it is important for the partners to establish a safe word. Safe words are unique and would not be connected to what is happening during the S/M experience. If a partner uses the safe word, it is the cue to stop. Because of the power, pleasures, and pain dynamics associated with S/M, words like "stop," "no," "don't," and "that hurts" are generally not used as safe words. Consider unique words not typically spoken in the bedroom or perhaps something easier for you and your partner(s) to remember, like silver, mango, peanut butter, or grandma.
If you're not sure where to draw the line for yourself, try fantasizing about it first before acting. If in your fantasies you go beyond your own limits of behavior, it's okay because that's exactly what fantasy is, a mental testing ground for limits. If your fantasies repel you because you are afraid you might act on them, this is important info about yourself and your limits. Remember, you are in control. If you're not in control, then it's not S/M.
At Columbia, Conversio Virium is a student BD (bondage, domination) SM discussion organization, who can be thanked for some of the information in this answer. You can send e-mail to email@example.com for further information about them. Outside the University, contact The Eulenspiegel Society, the oldest BDSM support group in the United States. As far as reading materials, try The Ultimate Guide to Kink: BDSM, Role Play and the Erotic Edge by Tristan Taormino.
Have fun and play safely!